1. Field of the Invention
This invention relates to electronic musical tone synthesizers and in particular is concerned with a digital tone generator producing an ensemble effect.
2. Description of the Prior Art
It is widely recognized that the tonal performance of an electronic musical instrument is enhanced by producing tones having an ensemble effect. The usual method of producing an ensemble effect is to generate two, or more, tones, whose fundamental frequencies differ by some small frequency difference. The motivation is to imitate the ensemble tone produced by a chorus of musical instruments which are not precisely in tune. It is this type of "out-of-tune" ensemble that produces the warm tone produced by a section of violins even when they are played in unison.
Various arrangements have been employed to electronically generate two simultaneous tones which are slightly detuned. The straightforward arrangement is to simply duplicate each tone generator and to use detuned clocks for each tone generator. Unfortunately the straightforward arrangement is usually an expensive solution because of the cost associated with duplicating the entire tone generation system. There is also the problem of using two clocks which must be only slightly out-of-tune and cannot be allowed to drift independently in frequency with changes in their ambient conditions.
Several arrangements have been developed for obtaining the two out-of-tune tone generators required for an ensemble effect by using only a single clock for both tone generators. Representative prior art is described in the following patents.
In U.S. Pat. No. 3,809,792, entitled "Production Of Celeste In A Computor Organ," apparatus is described for producing a celeste effect in a tone generator wherein the amplitudes of successive sample points of a musical waveshape are computed in real time. The amplitude of each sample point is obtained by summing two sets of Fourier components. One set is associated with the true pitch of the selected note and the second set is generated at a slightly higher pitch. The net result is a celeste-like effect, or an ensemble effect.
In U.S. Pat. No. 4,112,803 entitled "Ensemble And Anharmonic Generation In A Polyphonic Tone Synthesizer" a method is described for producing two out-of-tune musical tones. The first tone is produced in the manner described in U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer." The second tone is produced by using the same waveshape data as that computed for the first tone and repetitively reading out the data from a memory while slowly advancing the starting memory address. This action is equivalent to producing a linear phase shift which produces a tone at an increased frequency with respect to the note clock used to address the waveshape data from the memories. The net result is the production of two out-of-tune musical tones by using only a single note clock and a low frequency timing source.
In U.S. Pat. No. 4,205,580 entitled "Ensemble Effect In An Electronic Musical Instrument" a method is described for producing an ensemble effect in a tone generator of the type described in the above referenced U.S. Pat. No. 4,085,644. The waveshape data stored in the master data set are converted to analog signals by means of two digital-to-analog converters. The ensemble effect is produced by transferring data to the second converter at the same rate that data is transferred to the first converter but by having either one data point deleted or repeated in the second data set. Because the second data set has one less, or one extra, data point, the resulting musical tones from the two digital-to-analog converters change phase linearly with respect to each other with each successive cycle of the waveshape and thereby produces the desired detuning for an ensemble effect.